The important mosquito vector of yellow fever and dengue viruses, Aedes aegypti, is typically dependent on the man-altered environment. In Africa, however, two forms coexist. One is confined largerly to human cultures which practise water storage inside houses and the other is dependent on rain-filled breeding places. Where the two forms occur together their habitat selection is more sharply defined than where only one form occurs. If this evidence for competition between two more or less isolated forms is true it would have important implications for vector control and arbovirus epidemiology. It is proposed to analyze more deeply the differences between these strains at all levels from allozyme frequencies by electrophoresis to population cage studies under precise energy cnditions. Response to man versus animal host will be measured as a selective factor in the experimental populations. Relationships to the other species Aedes mascarensis and Aedes albopictus will be compared to intraspecific differences. Physiological responses of strains and species to varying environmental conditions will be compared for evidence of adaptive differences. A computer simulation which has been developed will be updated as new data becomes available and used to test the outcome of the popultion cage experiments..